DESCARTES: De-ESCAlation of RadioTherapy in Patients With Pathologic Complete rESponse to Neoadjuvant Systemic Therapy

Inclusion Criteria:

* Women, aged ≥ 18 years
* Invasive HR positive/Her2 negative, Her2+ (ER/PR +/-) or TN breast cancer
* Concurrent DCIS in pre-NST biopsy is allowed if there is no suspicion of extensive component i.e. absence of non-mass enhancement on pre-NST MRI, contrast-enhanced mammography or breast-specific gamma imaging and/or absence of calcifications on pre-NST mammography
* Primary tumour (T) clinical stage cT1-2
* Unifocal disease; confirmed by pre-NST MRI, contrast-enhanced mammography or breast-specific gamma imaging
* Clinical nodal stage 0; absence of lymph node metastases should be confirmed by ultrasound or FDG-PET/CT
* Neoadjuvant systemic treatment (NST)
* Marker placed in breast tumour prior to NST
* Breast conserving surgery performed, i.e. no mastectomy
* Sentinel node biopsy performed before or after NST
* Pathologic complete response in breast and lymph nodes, i.e. no residual tumour cells or DCIS detected
* Written informed consent

Exclusion Criteria:

* Primary tumour (T) clinical stage cT3-4
* Pre- or post-NST diagnosis of nodal disease including isolated tumour cells
* Patients without axillary ultrasound or FDG-PET/CT pre-NST
* History of breast cancer or DCIS
* Synchronous contralateral breast cancer or DCIS
* Synchronous M1 disease
* Carrier of gene mutation associated with increased risk of breast cancer, i.e. BRCA1, BRCA2, CHEK2, TP53 or PALB-2